Denitrification in recirculating systems

Organic carbon discharge from recirculating systems is reduced when endogenous carbon sources originating from the fish waste are used to fuel denitrification.. Denitrification in these

Denitrification in recirculating systems:

Theory and applications

Jaap van Rijna,* , Yossi Talb, Harold J Schreierb,c

a

Department of Animal Sciences, Faculty of Agricultural, Food and Environmental Quality Sciences,

The Hebrew University of Jerusalem, P.O Box 12, Rehovot 76100, Israel

b

Center of Marine Biotechnology, University of Maryland Biotechnology Institute, 701 E Pratt St., Baltimore, MD 21202, USA

c

Department of Biological Sciences, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA.

Received 31 January 2005; accepted 21 April 2005

Abstract

Profitability of recirculating systems depends in part on the ability to manage nutrient wastes Nitrogenous wastes in these systems can be eliminated through nitrifying and denitrifying biofilters While nitrifying filters are incorporated in most recirculating systems according to well-established protocols, denitrifying filters are still under development By means of denitrification, oxidized inorganic nitrogen compounds, such as nitrite and nitrate are reduced to elemental nitrogen (N2) The process is conducted by facultative anaerobic microorganisms with electron donors derived from either organic (heterotrophic denitrification) or inorganic sources (autotrophic denitrification) In recirculating systems and traditional wastewater treatment plants, heterotrophic denitrification often is applied using external electron and carbon donors (e.g carbohydrates, organic alcohols) or endogenous organic donors originating from the waste In addition to nitrate removal, denitrifying organisms are associated with other processes relevant to water quality control in aquaculture systems Denitrification raises the alkalinity and, hence, replenishes some of the inorganic carbon lost through nitrification Organic carbon discharge from recirculating systems

is reduced when endogenous carbon sources originating from the fish waste are used to fuel denitrification In addition to the carbon cycle, denitrifiers also are associated with sulfur and phosphorus cycles in recirculating systems Orthophosphate uptake

by some denitrifiers takes place in excess of their metabolic requirements and may result in a considerable reduction of orthophosphate from the culture water Finally, autotrophic denitrifiers may prevent the accumulation of toxic sulfide resulting from sulfate reduction in marine recirculating systems Information on nitrate removal in recirculating systems is limited to studies with small-scale experimental systems Packed bed reactors supplemented with external carbon sources are used most widely for nitrate removal in these systems Although studies on the application of denitrification in freshwater and marine recirculating systems were initiated some thirty years ago, a unifying concept for the design and operation of denitrifying biofilters in recirculating systems is lacking

# 2005 Elsevier B.V All rights reserved

Keywords: Denitrification; Recirculating aquaculture systems; Nitrate removal

www.elsevier.com/locate/aqua-online

* Corresponding author Tel.: +972 8 9489302; fax: +972 8 9465763.

E-mail address: vanrijn@agri.huji.ac.il (J van Rijn).

0144-8609/$ – see front matter # 2005 Elsevier B.V All rights reserved.

doi:10.1016/j.aquaeng.2005.04.004

1 Introduction

In most recirculating systems, ammonia (referring

to NH3 and NH4+) removal by nitrification, sludge

removal by sedimentation or mechanical filtration,

and water exchange are the vital forms of water

treatment (van Rijn, 1996) Often, 5–10% of the

system volume is replaced each day with new water to

prevent accumulation of nitrate and dissolved organic

solids (Masser et al., 1999) When comparing the

various biological processes important for water

quality control in regular fishponds with those in

recirculating systems, it can be concluded that

biological water treatment in the latter systems is

very limited In regular, earthen fishponds, inorganic

nitrogen levels in the water column are low, despite the

input of protein-rich supplementary feed Biological

removal of ammonia in these ponds takes place by

several biological processes: algal assimilation and

bacterial decomposition of algae, ammonification,

nitrification and denitrification (Shilo and Rimon,

1982; van Rijn et al., 1984; Diab and Shilo, 1986;

Hargreaves, 1998) Denitrification in these ponds is

confined to the sediments, where the presence of

anoxic conditions as a result of degradation of organic

matter and, in addition, the liberation of low molecular

weight carbon compounds, provide suitable

condi-tions for denitrification (Diab and Shilo, 1986;

Avnimelech et al., 1992; Hopkins et al., 1994;

these conditions in recirculating systems, by

com-partmentalization of each of the above nitrogen

transformation processes, is essential for reducing

water consumption and environmental impact of these

systems

Nitrate reaches high concentrations in recirculating

systems where nitrifying biofilters are used for

ammonia removal Reported maximum values of

nitrate in recirculating systems are as high as 400–

500 mg NO3-N/l (Otte and Rosenthal, 1979; Honda

recirculating systems and are dictated mainly by water exchange rates and the extent of nitrification and nitrate removal Contrary to ammonia and nitrite, nitrate is relatively non-toxic to aquatic organisms However, high nitrate concentrations can affect the growth of commercially cultured aquatic organisms, such as: eel (Kamstra and van der Heul, 1998), octopus (Hyrayama, 1966), trout (Berka et al., 1981) and shrimp (Muir et al., 1991) Increased efforts are now directed toward nitrate control in recirculating systems Apart from the direct toxic effect on fish, nitrate removal is conducted for other reasons in recirculating systems: (1) environmental regulations associated with effluent discharge have permissible nitrate levels as low as 11.3 mg NO3-N/l (European Council Directive, 1998); (2) prevention of high nitrite levels resulting from incomplete ‘‘passive’’ nitrate reduction; (3) stabilization of the buffering capacity; and (4) the concomitant elimination of organic carbon, orthophosphate and sulfide from the culture water during biological nitrate removal

In this review, biological pathways of nitrate removal are discussed as well as links between denitrifying organisms and carbon, phosphate and sulfur cycles in recirculating systems Applications of biological nitrate removal in recirculating systems are reviewed Finally, the anammox process, an alter-native pathway for ammonia and nitrate removal, is discussed

2 Biological nitrate removal Biological nitrate removal is conducted by a wide variety of organisms by either assimilatory or dissimilatory pathways (Table 1) Organisms capable

of assimilatory nitrate reduction use nitrate, rather Table 1

Biological nitrate reductiona

Assimilatory nitrate reduction (NO 3
! NO 2
! NH 4

+

+

Plants, fungi, algae, bacteria Dissimilatory nitrate reduction

Dissimilatory nitrate reduction to ammonia (NO 3
! NO 2
! NH 4+) O 2 , C/N Anaerobic and facultative anaerobic bacteria Denitrification (NO 3
! NO 2
! NO ! N 2 O ! N 2 ) O 2 , C/N Facultative anaerobic bacteria

a

From van Rijn and Barak (1998) , adapted from Tiedje (1990)

than ammonia, as a biosynthetic nitrogen source In

most organisms, this process occurs in the absence of

more reduced inorganic nitrogen species (e.g

ammo-nia) Assimilatory nitrate reduction takes place under

aerobic as well as anaerobic conditions No net

removal of inorganic nitrogen is accomplished by this

process, since inorganic nitrogen is converted to

organic nitrogen

Dissimilatory nitrate removal refers to the

reduc-tion of nitrate to more reduced inorganic nitrogen

species with the concomitant release of energy The

dissimilatory pathway is employed mainly by two

groups of prokaryotic organisms Nitrate is reduced to

either nitrite or ammonia by one group, and the other

group reduces nitrate via nitrite to gaseous nitrogen

forms with elemental nitrogen (N2) as the end product

The former process, dissimilatory nitrate reduction to

ammonia (DNRA), is conducted by fermentative

bacteria using nitrate as a final electron acceptor when,

for bioenergetic reasons, reduction of organic matter

(fermentation) is not possible (Tiedje, 1990)

Deni-trifiers represent the second group of dissimilatory

nitrate reducers and comprise a wide array of

prokaryotic organisms Most of these organisms are

facultative anaerobes and use nitrate as a final electron

acceptor in the absence of oxygen Elemental nitrogen

is the end product of this process, but intermediate

accumulation of nitrite, nitric oxide and nitrous oxide

may take place under certain conditions

Hetero-trophic denitrifiers, using organic carbon compounds

as a source of biosynthetic carbon and electrons, are

the most common denitrifiers in nature In some

reduced environments low in dissolved carbon,

autotrophic denitrifiers are the prevalent denitrifiers

using reduced inorganic compounds, such as Mn2+,

Fe2+, sulfur and H2as electron sources and inorganic

carbon as a biosynthetic carbon source (Korom, 1992)

Environmental factors, in particular the availability

and type of organic carbon compounds and the

oxidation/reduction state of the aquatic environment,

dictate to a large extent the occurrence dissimilatory

nitrate reducers High C/N ratios (Tiedje, 1990) and

high sulfide concentrations (Brunet and Garcia-Gil,

1996) in the environment are thought to favor DNRA

organisms over denitrifiers Among the denitrifiers,

the type and quantity of organic carbon compounds

influences the accumulation of intermediate products,

such as nitrite and inorganic nitrogen gases (

Nishi-mura et al., 1979; NishiNishi-mura et al., 1980; van Rijn and Sich, 1992; Blaszczyk, 1993; van Rijn et al., 1996) Oxygen is an important regulator of denitrification Although aerobic denitrification has been reported (Robertson and Kuenen, 1984), most denitrifiers are facultative anaerobes and reduce nitrate in the absence

of oxygen Incomplete reduction of nitrate to intermediate products occurs at low oxygen concen-trations due to differential repression of oxygen on enzymes involved in the nitrate reduction pathway (Betlach and Tiedje, 1981) Oxygen repression often is accompanied by nitrite accumulation in the aquatic medium (van Rijn and Rivera, 1990) Other environ-mental factors that repress denitrification activity and cause nitrite accumulation are: sub-optimal pH values (Beccari et al., 1983; Thomsen et al., 1994; Almeida

et al., 1995) and high light intensities (Barak et al.,

1998)

3 Heterotrophic versus autotrophic denitrification

3.1 Heterotrophic denitrification Heterotrophic denitrifiers derive electrons and protons required for nitrate reduction to elemental nitrogen from organic carbon compounds Such compounds include carbohydrates, organic alcohols, amino acids and fatty acids For example, utilization

of acetate as a carbon source for denitrification proceeds as follows:

5CH3COO
þ 8NO3
þ 3Hþ

! 10HCO3
þ 4N2ðgÞ þ 4H2O (1) The C/N ratio required for complete nitrate reduction to nitrogen gas by denitrifying bacteria depends on the nature of the carbon source and the bacterial species (Payne, 1973) For most readily available organic carbon sources, a COD/NO3
-N (w/w) ratio from 3.0 to 6.0 enables complete nitrate reduction to elemental nitrogen (Montieth et al., 1979; Narcis et al., 1979; Skinde and Bhagat, 1982), where COD stands for chemical oxidation demand and is expressed as mgO2/l As noted above, carbon limitation will result in the accumulation of inter-mediate products, such as NO and N O, while excess

carbon will promote dissimilatory nitrate reduction to

ammonia In addition, denitrification rates depend on

the type of carbon source In anaerobic reactors, for

example, denitrification was faster with acetate than

glucose or ethanol (Tam et al., 1992) Differences in

denitrification rates were found when denitrifying

isolates from a fluidized bed reactor in a recirculating

system were incubated with different short-chain

volatile fatty acids (Aboutboul et al., 1995) In

wastewater treatment plants and aquaculture systems,

exogenous carbon substrates often are used to drive

denitrification, with methanol most often used (Payne,

1973) However, endogenous carbon compounds

liberated from organic sludge digestion may be used

for this purpose in recirculating systems (Aboutboul

et al., 1995)

3.2 Autotrophic denitrification

In addition to organic carbon, some denitrifying

bacteria may use inorganic compounds, such as

hydrogen and reduced sulfur, manganese and iron

species as electron donors Few studies have

demon-strated the use of these processes to remove nitrate

from contaminated water, but a sulfur-limestone

reactor was used to promote autotrophic

denitrifica-tion from wastewater (Flere and Zhang, 1998; Zhang

and Lampe, 1999) The feasibility of denitrification at

low COD/N ratios was demonstrated by taking

advantage of the symbiotic relationship between

sulfur denitrifying bacteria and sulfate reducing

bacteria (Kim and Son, 2000) Some advantages of

autotrophic denitrification over heterotrophic

denitri-fication include: (1) low biomass buildup (biofouling)

and reduction of reactor clogging and (2) avoidance of

organic carbon contamination of treated water

4 Denitrifiers and phosphate removal

Enhanced biological phosphorus removal (EBPR)

from domestic wastewater in activated sludge plants is

accomplished by alternate stages, where sludge is

subjected to anaerobic and aerobic conditions

Phosphorus is released from bacterial biomass in

the anaerobic stage and is assimilated by these bacteria

in excess as polyphosphate (poly-P) during the aerobic

stage Phosphorus is removed from the process stream

by harvesting a fraction of the phosphorus-rich bacterial biomass (Toerien et al., 1990) Some of these polyphosphate accumulating organisms (PAO) are also capable of poly-P accumulation under denitrifying conditions, i.e with nitrate instead of oxygen serving as the terminal electron acceptor (Barker and Dold, 1996; Mino et al., 1998) Studies on poly-P accumulating organisms have revealed the involvement of specific metabolic properties under anaerobic, aerobic and anoxic conditions (Mino et al.,

1998) Under anaerobic conditions, acetate or other low molecular weight organic compounds are con-verted to polyhydroxyalkanoates (PHA), poly-P and glycogen are degraded and phosphate is released Under aerobic and anoxic conditions, PHA is converted to glycogen, phosphate is taken up and poly-P is synthesized intracellularly Under the latter conditions, growth and phosphate uptake is regulated

by the energy released from the breakdown of PHA Some heterotrophic denitrifiers exhibit phosphorus storage in excess of their metabolic requirements through poly-P synthesis under either aerobic or anoxic conditions, without the need for alternating anaerobic/aerobic switches (Barak and van Rijn,

to use PHA as an energy source for poly-P synthesis and derived energy from oxidation of external carbon sources The feasibility of this type of phosphate removal was demonstrated for freshwater as well as marine recirculating systems (Barak and van Rijn, 2000b;Shnel et al., 2002; Barak et al., 2003; Gelfand

et al., 2003) In the culture water of these systems, stable orthophosphate concentrations were found throughout the culture period Phosphorus immobili-zation took place in the anoxic treatment stages of the system where it accumulated to up to 19% of the sludge dry weight

5 Alkalinity control by denitrification

In recirculating systems, intensive nitrification leads

to an alkalinity loss and a resulting pH decline of the culture water Acidic conditions negatively impact the biofilter performance and alkalinity supplements, such

as sodium bicarbonate are routinely administered to stabilize pH and alkalinity Heterotrophic denitrifica-tion results in an alkalinity gain and by incorporating

this process in the treatment scheme of a recirculating

system one might be able to eliminate or reduce the use

of alkalinity supplements (van Rijn, 1996)

The amount of acid required to titrate the bases in

water is a measure of the alkalinity of water A

chemical reaction producing acid will lower the

alkalinity of the water, while the opposite holds for a

reaction in which acid is consumed or hydroxyl ions

are produced During nitrification, alkalinity decreases

by approximately 7 mg CaCO3 for each mg of

ammonia-N oxidized to nitrate according to the

following simplified stoichiometry:

NH4þþ 2O2¼ NO3
þ 2Hþþ H2O (2)

(Alkalinity loss = 2 meq of alkalinity per mole NH4

or 7.14 mg CaCO3/mg NH4 -N)

Some of this alkalinity loss is regained when, in

addition to nitrification, denitrification is used as a

water treatment stage Heterotrophic denitrification

causes a release of hydroxyl ions and raises alkalinity

Each mg of nitrate-N reduced to N2 causes an

alkalinity increase of 3.57 mg CaCO3according to the

following stoichiometry:

2NO3
þ 12Hþþ 10e
¼ N2þ 6H2O (3)

(Alkalinity gain = 1 meq of alkalinity per mole NO3or

3.57 mg CaCO3/mg NO3
-N)

Autotrophic denitrification on reduced sulfur

compounds may generate or consume alkalinity

depending on the reduced sulfur species oxidized

(Oh et al., 2001; Kleerebezem and Mendez, 2002) In

marine systems, reduced sulfur species are often

produced by reduction of sulfate, an

alkalinity-generating process (Eq (4)) Sulfate reduction in

combination with oxidation of reduced sulfur

com-pounds will cause an overall increase in alkalinity as

illustrated by the reduction of sulfate to sulfide and its

subsequent reoxidation to sulfate (Eqs.(4) and (5))

SO4
þ 10Hþþ 8e
! H2S þ 4H2O (4)

(Alkalinity gain = 2 meq of alkalinity per mole SO4

or 100 mg CaCO3/mole SO4
)

5H2S þ 8NO3
! 5SO4
þ 4N2þ 4H2Oþ 2Hþ

(5)

(Alkalinity loss = 2 meq per 5 moles H2S or 20 mg CaCO3/mole H2S)

Sulfate reduction to sulfide generates an alkalinity

of 100 mg CaCO3 per mole SO42
(Eq (4)), and sulfide driven nitrate reduction to N2 consumes an alkalinity of 20 mg CaCO3/mole H2S (Eq.(5)) Like heterotrophic denitrification, the coupled process of sulfate reduction, sulfide oxidation and nitrate reduc-tion results in a net alkalinity generareduc-tion of 400 mg CaCO3per 8 moles of NO3reduced or 3.57 mg CaCO3 per mg NO3-N reduced

6 Denitrification in recirculating aquaculture systems

In the following section, a distinction is made between passive and induced denitrification Fresh-water and marine recirculating systems are discussed separately as are polymer-based, denitrification reactors used in aquariums A summary of applica-tions of denitrification reactors in recirculating systems is presented in Table 2 and denitrification rates by some of these reactors, discussed in the last part of this section, are presented inTable 3 6.1 Passive denitrification in recirculating systems

Denitrification occurs in anoxic environments in the presence of oxidized carbon and inorganic nitrogen compounds Given these requirements, it might be assumed that such conditions, confined to specific microsites, exist in most recirculating aqua-culture systems In a study on trickling filter biofilms, denitrification activity was observed in distinct zones

of the biofilm (Dalsgaard and Revsbech, 1992)

By means of microsensors, denitrification activity was measured at a depth of 0.2–0.3 mm below the biofilm surface Oxygen levels and organic matter availability dictated the depth of the denitrifying zone Ammonia lowered nitrate assimilation rates and increased nitrate availability for denitrification Few studies have quantified passive denitrifying activity

in recirculating systems Passive denitrification, estimated by mass and isotopic balances of major nitrogen pools (Thoman et al., 2001), accounted for a

Table 2

Denitrification reactors in recirculating systems

Denitrifying reactor Organism(s) cultured Carbon/electron donor Reference

Freshwater systems

Activated sludge Tilapia, eel Glucose/methanol Otte and Rosenthal (1979) Activated sludge Trout Hydrolyzed corn starch Kaiser and Schmitz (1988) Digestion basin and fluidized bed reactor Tilapia Endogenous van Rijn and Rivera (1990) ,

Arbiv and van Rijn (1995) , Shnel et al (2002)

Polymers Ornamental fish Biodegradable polymers Boley et al (2000)

Marine systems

Packed bed reactor Atlantic and Chinook salmon Methanol Balderston and Sieburth (1976) Packed bed reactor Japanese Flounder Glucose Honda et al (1993)

Fluidized bed reactor Ornamental fish Methanol Grguric and Coston (1998)

Packed bed reactor Ornamental fish Methanol Grguric et al (2000a,b) Packed bed reactor Shrimp Ethanol/methanol Menasveta et al (2001)

Digestion basin and fluidized bed reactor Gilthead seabream Endogenous Gelfand et al (2003) Moving bed bioreactor Gilthead seabream Starch Morrison et al (2004)

Table 3

Volumetric denitrification rates by some denitrifying reactors

Denitrifying reactor Medium Carbon source Nitrate removal

rate (mg NO 3 -N/l/h)

Reference Freshwater systems

Packed bed Biodegradable polymers PHB (C 4 H 6 O 2 ) n 7–41 Boley et al (2000) Packed bed Biodegradable polymers PCL (C 6 H 10 O 2 ) n 21–166 Boley et al (2000) Packed bed Biodegradable polymers Bionolle (C 6 H 8 O 4 ) n 1.5–77 Boley et al (2000)

Packed bed Freeze-dried alginate beads Starch 26.0 Tal et al (2003)

Marine systems

Packed bed Porous medium Methanol 7.3–8.4 a Grguric et al (2000a, b)

Packed bed Plastic balls/crushed oyster shells Ethanol/methanol 6.6a Menasveta et al (2001) Packed bed Freeze-dried alginate beads Starch 2.6 Tal et al (2003)

Moving bed reactor Plastic medium Endogenous 24.0 Tal and Schreier (2004)

a

Extrapolated (rates were not provided by authors).

nitrogen loss of 9–21% in a closed recirculating

mariculture system for culture of red drum (Sciaenops

ocellatus) These findings were supported by a study

on a marine recirculating shrimp production system

mem-brane inlet mass spectrometry, significant nitrate

removal was detected in media from a nitrifying filter

and sediment derived from the system Additional

evidence for the denitrification potential of nitrifying

media was recently provided in a study on a moving

bed bioreactor in a recirculating facility for culture of

gilthead seabream (Sparus aurata) byTal et al (2003)

6.2 Induced denitrification in freshwater

recirculating systems

Studies on these reactors were initiated in Germany

byMeske (1976)by incorporating an activated sludge

tank in an experimental recirculating culture system for

common carp (Cyprinus carpio) Similar experimental

systems with or without addition of external carbon

sources were subsequently operated by a number of

investigators with different freshwater fish (Otte and

Rosenthal, 1979; Gabel, 1984; Kaiser and Schmitz,

1988; Schmitz-Schlang and Moskwa, 1992; Knosche,

1994) Denitrifying activity in packed bed columns was

studied byAbeysinghe et al (1996)andSuzuki et al

Denitrification on endogenous carbon sources was

studied in a closed freshwater recirculating culture

system for tilapia (Arbiv and van Rijn, 1995; van Rijn

and Barak, 1998; Shnel et al., 2002) In these studies,

carbon compounds, released from the breakdown of

endogenous carbon, were used to fuel denitrification in

an anoxic treatment step consisting of a digestion basin

and a fluidized bed reactor The feasibility of using

endogenous fermentation generated carbon sources for

denitrification in recirculating aquaculture systems also

was described byPhillips and Love (1998)

6.3 Induced denitrification in marine

recirculating systems

Pioneer work on marine closed systems for the

culture of salmonids was conducted by Meade and

coworkers (Meade, 1973; Meade, 1974; Meade and

was examined by Balderston and Sieburth (1976)

using experimental packed columns fed with metha-nol A spin-off system is successfully used in a recirculating marine culture system for cephalopods (Whitson et al., 1993; Lee et al., 2000) Packed bed reactors fed with different external carbon sources were used in a number of other studies with different marine organisms (Honda et al., 1993; Sauthier et al.,

moving bed bioreactors were used for denitrification

in a gilthead seabream (Sparus aurata) recirculating system (Morrison et al., 2004) Large denitrification units for treatment of public aquarium water at the New Jersey State Aquarium (total aquarium volume: 2.9 million l) and the Living Seas at EPCOT Center, Florida (total aquarium volume: 23 million l) have been employed successfully in recent years Deni-trification is induced in these systems using submerged and fluidized bed reactors with addition of methanol (Grguric and Coston, 1998; Grguric et al., 2000a,b) The feasibility of denitrification in a marine recirculat-ing system for culture of gilthead seabream with endogenous carbon as the sole carbon source was demonstrated in a closed system comprising an anoxic digestion basin and fluidized bed reactor (Gelfand et al.,

2003) Nitrate removal in this system was mediated by both heterotrophic and autotrophic denitrification Chemical analyses of the sulfur transformations and microbiological analyses of the bacterial populations in this treatment system revealed that sulfide, produced by sulfate reduction in the anaerobic parts of the digestion basin, was reoxidized by autotrophic denitrifiers (Cytryn et al., 2003) It is interesting to note that alkalinity lost in the nitrifying treatment stage was fully regained in the anoxic treatment stage (Gelfand et al.,

2003) A recirculating system for culture of gilthead seabream with nitrate removal by autotropic denitrifiers

on reduced sulfur compounds was recently reported by

Tal and Schreier (2004) 6.4 Denitrification by means of immobilized systems

Nitrate removal by means of immobilized deni-trifiers has been studied since the 1980s (Nilson et al.,

1980) Entrapment of denitrifiers is accomplished with non-synthetic materials, such as agar, k-carrageenan, chitosan and alginate, or synthetic polymers, such as PVC—polyvinylchloride, PP—polypropylene and

PS—polystyrene with or without addition of a

degradable carbon source (Tal et al., 2001)

Biode-gradable polymers, serving both as matrix and carbon

source, are also used for this purpose (Biedermann

complexes has been studied only on an experimental

scale in aquariums.Nagadomi et al (1999)performed

tests on nitrate removal in aquariums stocked with

ornamental carp by means of the photosynthetic

bacterium, Rhodobacter spaeroides S, immobilized in

alginate and polyvinyl alcohol (PVA) gel beads PVA

gels were also used by Park et al (2001) with

immobilized denitrifiers derived from activated sludge

in a study on nitrate removal in marine recirculating

aquarium systems.Tal et al (2003a,b)used a

freeze-dried, alginate-starch matrix as an entrapping agent

for heterotrophic denitrifiers (Pseudomonas spp.)

in the removal of nitrate from freshwater and

marine aquariums A different approach was used

in a study byBoley et al (2000)where several types

of biodegradable polymers were used a substrate

for endemic denitrifiers in a freshwater aquarium

system

6.5 Denitrification rates

Oxidation of an organic carbon and electron donor

and subsequent reduction of nitrate to elemental

nitrogen yields around 70% of the energy gained with

oxygen as the final electron acceptor (Payne, 1970)

High nitrate removal rates can be accomplished with

this energy efficient process under suitable conditions

As stated earlier, information on denitrification in

recirculating systems is scarce and nitrate removal rates

by denitrification reactors are reported in only few

studies In some studies, sufficient information is

provided to allow calculation of these rates, while in

others this information is lacking Volumetric nitrate

removal rates by different denitrifying reactors used in

aquaculture facilities and in aquariums are summarized

inTable 3 The wide range (1–166 mgNO3-N/l/h) in

rates is most likely due to differences in operational

parameters, such as system configuration, types of

electron donor, reduction states of the reactors, and the

ambient nitrate concentrations at which the various

reactors were operated No clear differences in

denitrification rates are found between systems in

which external carbon sources are used to fuel

denitrification and systems that are operated with endogenous carbon sources Also, no distinct differ-ences are found between denitrification reactors operated in freshwater and marine systems It should

be noted, however, that due to differences in operational parameters of these systems, such comparisons are extremely difficult

The reported volumetric nitrate removal rates do provide an indication for the size of denitrification reactors relative to that of nitrification reactors Volumetric ammonia removal rates in commonly used nitrification filters, such as bead filters and trickling towers are 1.4–15 mg TAN/l/h and 3–4 mg TAN/l/h, respectively (calculated from Timmons et al., 2001) These values are often lower than reported nitrate-nitrogen removal rates (Table 1), implying that nitrate removal can be accomplished in smaller reactors than ammonia removal This finding might be explained by the different requirements of both processes Nitrifying filters are characterized by a relatively large void volume in order to prevent organic matter accumulation and optimal oxygen penetration into the nitrifying biofilm This is in contrast to denitrifying reactors, which can be designed in a more compact manner due to their anaerobic mode of operation In addition to size, daily water flow through nitrification and denitrification reactors differs significantly due to differences in allowable ammonia and nitrate concentrations in the culture systems The need for low ambient ammonia concentrations requires a rapid water exchange between fish tanks and nitrification reactors, coinciding with relatively low ammonia removal rates per single filter pass System operation at relatively high ambient nitrate concentrations supports relatively high nitrate removal rates per single reactor pass and allows a much smaller water exchange between fish tanks and denitrification reactors

7 Anammox as an alternative to denitrification Anaerobic ammonia oxidation (anammox) is a microbially-mediated process (Mulder et al., 1995) identified in engineered systems as well as in natural environments, and has been applied to wastewater treatment systems (Schmidt et al., 2003) Carried out

by bacteria of the order Planctomycetales, anammox eliminates nitrogen by combining ammonia and nitrite

to produce nitrogen gas (van de Graaf et al., 1995),

thereby providing an alternative approach to nitrogen

removal via denitrification Application of anammox

in treating recirculating system water is desirable as it

has the potential of providing significant oxygen and

energy savings due the oxidation of only half of the

ammonia produced in the system (Eqs (6)–(8))

Moreover, anammox enables complete ammonia

removal via autotrophic pathways without the

requirement of organic carbon

Partial nitrification : 2NH4 þþ 1:5O2

! NH4 þþ NO2
þ H2Oþ 2Hþ (6)

Anammox : NH4 þþ NO2
! N2þ 2H2O (7)

Total : 2NH4 þþ 1:5O2! N2þ 3H2O þ 2Hþ

(8)

In studies designed to characterize the microbial

consortium of aerobic and anaerobic biofilters, Tal

et al (2003b, 2004)obtained evidence for the presence

of anammox-related microorganisms in aquaculture

recirculating systems Using molecular identification

methods based on 16S-rRNA gene sequences,

anammox bacterial 16S rRNA sequences were

amplified from the microbial consortia of the filters

from both marine and freshwater recirculated

aqua-culture systems (Tal et al., 2004) Anammox activity

was demonstrated in lab-scale experiments by

incubating microbial consortia under anaerobic

con-ditions in the presence of ammonia and nitrite While

the actual portion of nitrogen released via anammox is

difficult to assess, it is reasonable to consider that

some of the ‘‘passive denitrification’’ or nitrogen loss

observed in recirculating systems could be explained

by anammox Whether anammox could be applied to

recirculating systems as a means to control nitrogen

load in lieu of conventional denitrification approaches

remains to be determined A major limitation of the

anammox process is the slow growth rate for these

bacteria With doubling times of around 11 days

(Strous et al., 1999a,b), it seems unlikely that these

organisms can be enriched in biofilter systems

Nevertheless, recent reports on the successful

applica-tion of anammox in wastewater treatment plants

encouraging and justify studies on the exploitation

of this process in aquaculture systems

8 Future directions concerned with denitrification in recirculating systems Research on denitrification in recirculating systems has been conducted for a considerable time Often, these studies were performed on laboratory simulation systems or small, experimental facilities These systems can only partially simulate conditions in commercial recirculating systems, and a need exists for information on the performance of denitrifying reactors in whole systems Even before the imple-mentation of a denitrification treatment step, basic studies on nutrient budgets, such as those byThoman

recirculating systems and, more extensively, for pond systems (e.g Krom et al., 1985; Schroeder, 1987;

design of a denitrification reactor should be based on comprehensive understanding of the dynamics of nitrogen, carbon and other inorganic nutrients in a particular recirculating system Internal versus exter-nal carbon and electron donors for induction of denitrification as well as induction of heterotrophic or autotrophic denitrifiers should be based on rational rather than arbitrary information Denitrification combined with organic matter digestion enables a virtually closed operation of freshwater and marine recirculating systems Enabling the culture of marine species away from the coast will direct the aquaculture industry to new, unexplored avenues (Krom et al.,

2001)

At present, application of denitrification in com-mercial recirculating systems is conducted at a limited scale Based on the experimental systems reviewed in this paper, it seems that full scale implementation of denitrification is feasible However, the lack of studies

on large-scale recirculating systems, as mentioned above, has limited commercial application of deni-trification in recirculating systems Moreover, incen-tives to implement denitrification in commercial recirculating systems are still lacking Economic incentives related to savings on water usage, pH control and environmental discharge fees are still of inadequate significance in the total operation costs to necessitate

nitrate removal in these systems Illustrative of this

point is the fact that large scale denitrification, applied

in public aquariums (Grguric and Coston, 1998;

Grguric et al., 2000a,b), probably is based less on

financial considerations than system performance and

environmental impact The fact that little or no

documentation exists on the performance of

denitrifi-cation reactors in the few commercial systems using

this technology is another drawback for full-scale

application of denitrification Finally, like any new

technology, information transfer from experimental

facilities to commercial applications is time consuming

and requires cooperation to enable exchange of

information on benefits of the new technology

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